Yōkai

Yōkai(妖怪, Yōkai, lit meaning=Ghost, Phantom, Strange apparition) is a term used for wide variety of supernatural creatures from Japanese folklore. They comprise the majority of the series' cast and are also commonly referred to as "spirits." Some possess part-animal and part-human features (e.g. Kappa and Tengu), some were once humans who became yōkai after death (e.g. Gyūki and Kubinashi), and some are inanimate objects who came to life (Tsukumogami). Yōkai generally have a sort of spiritual or supernatural power and significantly longer lifespans than humans. Occasionally, the term Ayakashi is used instead of "yōkai," but the two terms are interchangeable. In the Past Arc, it was more common to use the former, whereas in the present day, the latter is more common.

Osore
is the term that denotes the unique skills and traits of each yōkai. It refers to the "fear" of the unknown, an emotional reaction produced when the yōkai represent themselves as "monsters". As yōkai first existed as creatures who induce fear in humankind, the general concept of "fear" revolves around being feared and respected by humans and making them feel small and weak. It involves exerting a wall of pressure to make one's presence feel larger than the actuality. When done correctly, this also creates a change in the mood and surrounding air - as seen whenever a dense fog appears when a Hyakki Yakō gathers in the series. Itaku states that Osore only applies to scaring humans.

Hyakki Yakō
is a term used for the gathering of yōkai beneath a leader's banner, which can only be mobilized by the leader's command.